1. Field of the Invention
The present invention relates to a process for producing 1,4-dihydroanthraquinones. More particularly, it relates to a process for producing 1,4-dihydroanthraquinones by oxidizing 1,4,4a,9a-tetrahydroanthraquinone or 1,4-dihydroanthrahydroquinone or an inert substituted derivative.
2. Description of the Prior Art
1,4-Dihydroanthraquinones are important starting materials for producing agrochemicals, dyes and other fine chemicals and also useful as agrochemicals by themselves.
Heretofore, it has been known to produce 1,4-dihydroanthraquinone (1,4-DHAQ) by several processes as follows.
(1) 1,4,4a,9a-Tetrahydroanthraquinone (THAQ) is oxidized in ethanol with ferric chloride; PA1 (2) THAQ is oxidized with ferric chloride in the presence of a wetting agent (Chemical Abstract 56, 7237.sup.e (1962)); PA1 (3) THAQ is oxidized with potassium bromate (Chemical Abstract 52, 12830.sup.b (1958)); PA1 (4) THAQ is oxidized with a gaseous oxygen in an aqueous medium at pH of 4 to 8 preferably 6 to 7 at 85.degree. to 100.degree. C. preferably 90.degree. to 95.degree. C. (B. Pat. No. 896,911 (Mar. 23, 1962)).
In the processes (1) and (2), expensive ferric chloride is used to cause the high cost. The process (3) is an experimental process and is not an industrial process. The process (4) is superior to the other processes as an industrial process. However, the reaction velocity is remarkably low to need longer than 4 hours and the purity of the resulting 1,4-DHAQ is 96% as maximum.
The inventors have studied various process for producing 1,4-DHAQ having high purity from THAQ with industrial advantages. As the result, the inventors have found that when the pH is higher than that of the process (4) and the temperature is lowered in the case of the oxidation of THAQ with molecular oxygen in an aqueous medium, the reaction is accelerated and the reaction time is remarkably shortened and the amount of the by-product of anthraquinone can be reduced, surprisingly.